404 G. O. Batzli et al. 



them is more extensive (see Batzli 1975a for review), ideas regarding their 

 impact on tundra are more complete. The impacts can be considered in 

 relation to three interacting components of the ecosystem, viz. microtop- 

 ography, vegetation and soils. While some inferences can be drawn re- 

 garding the Prudhoe Bay region, they are largely speculative. 



Lemmings at Barrow construct their burrows on elevated sites, e.g. 

 centers of high-centered polygons or rims of low-centered polygons, 

 which have favorable drainage. As a result, much of the drier tundra is 

 riddled with burrows, and barren areas are formed at burrow entrances 

 by deposition of soil. Runways connect the burrows, and hummocks 

 often develop between runways owing to erosion of fine materials that 

 are not stabilized by vegetation. In areas denuded by frost heaving, there 

 seems to be progressively greater hummock development associated with 

 increasing vegetation growth and lemming activity. Unfortunately, this 

 sequence has not been documented by long-term observations in one 

 area. 



In the Prudhoe Bay region, where lemmings are less abundant, their 

 impact is not as clear, although burrows and runways may be conspicu- 

 ous in some areas. Ground squirrel diggings on river banks, sand dunes 

 and pingos do create dramatic series of holes and mounds, but ground 

 squirrels occupy only a small portion of the tundra. Caribou generally 

 spread out as they graze, so compaction of soil caused by their trampling 

 is not obvious except for systems of trails leading to sand dunes where 

 they seek refuge from insect attack. Of course, the effects of grazers on 

 microtopography presumably are reflected in the soil characteristics, 

 such as soil temperature and depth of thaw. 



Exclosure studies at Barrow, originally started in 1950, indicate that 

 the elimination of lemming grazing causes several changes in vegetation 

 (Batzli 1975b). At well-drained sites, carpets of mosses and lichens devel- 

 op and graminoids become sparse. In low, wet sites graminoids continue 

 to dominate, but standing dead material accumulates and productivity 

 declines. Apparently, heavy lemming grazing disrupts mosses and li- 

 chens, which recover slowly. Graminoids, however, have their meristem- 

 atic tissue under the moss layer and can replace shoots rapidly by draw- 

 ing upon reserves in underground rhizomes. Chronic grazing by caribou 

 during summer may have less effect on vegetation than the intensive 

 grazing during winter and spring when lemming populations are high. 



Grazers can affect vegetation indirectly as well as directly (Batzli 

 1975b). The bulbet saxifrage, Saxifraga cernua, appears to be concen- 

 trated around old lemming burrows and trails as does an acrocarpous 

 moss, Funaria polaris (^. Murray, pers. comm.). Saxifraga is probably 

 there because lemmings disperse their sticky bulbets, while Funaria may 

 simply specialize on lemming feces as a substrate. Herds of caribou may 

 have significant local effects owing to trampling and deposition of man- 



